This invention relates to molten carbonate fuel cells and, in particular, to an electrolyte matrix for use in such cells.
Molten carbonate fuel cell (MCFC) power plants offer a highly efficient and environmentally clean option for power generation. A key component in a MCFC is the electrolyte matrix that provides both ionic conduction and gas sealing. The conventional electrolyte matrix is a porous, unsintered .gamma.-LiAlO.sub.2 ceramic powder bed impregnated with molten alkali carbonate electrolyte. It is essential for the electrolyte matrix to remain substantially crack or defect free to provide effective gas sealing during MCFC operations.
During such operation, the matrix experiences both mechanical and thermal stresses, and these stresses are believed to be the major contributing factors in causing matrix cracking. The difference in mechanical characteristics of active and wet seal areas of the matrix can cause high mechanical stress on the matrix. The thermal expansion mismatch between the carbonate electrolyte and ceramic particles, in addition to the large change in electrolyte volume associated with freezing/melting, causes high thermal stress on the matrix.
More particularly, the difference in thermal expansion coefficients between the LiAlO.sub.2 ceramic particles (less than 10.times.10.sup.-6 /.degree.C.) and the carbonate electrolyte (greater than 20.times.10.sup.-6 /.degree.C.) can generate a significant amount of compressive stress on the LiAO.sub.2 particles and tensile stress on the carbonates during cooling. For a temperature decrease of .about.400.degree. C. (from the 488.degree. C. solidification temperature of the Li/K eutectic carbonate to ambient temperature), this thermal stress can exceed 3,000 psi. This is much greater than the strength of the frozen carbonates (approximately 2,000 psi). Therefore, even in the absence of mechanical stress, the thermal stress generated by the thermal expansion mismatch is sufficient to cause cracking of the matrix during thermal cycling. Accordingly, the electrolyte matrix, without any additional second-phase reinforcement, can develop thru-cracks after only one thermal cycle due to high thermal stress.
The matrix can also experience cracking during initial MCFC stack start-up. During this period, organic binders in the matrix are removed (burnt off), nearly completely at about 350.degree. C. The resulting binderless matrix of unsintered ceramic powder bed is very weak in structure (even weaker than the matrix impregnated with carbonate during the 650.degree. C. operation) before impregnation by carbonate above the carbonate melting temperature (.about.488.degree. C. for the Li/K eutectic). Therefore, the matrix is very susceptible to mechanical or thermal cracking in the 350.degree. C. to .about.500.degree. C. start-up temperature range. Such cracks can propagate to large sizes during MCFC operation.
The matrix cracking during either start-up or operation permits undesired fuel and oxidant gas cross leakage, causing lower power generation efficiency, shorter life and, more importantly, poor power plant economy. Therefore, providing strong and tough matrices resistant to cracking to maintain good sealing capability is highly desired.
Various approaches for improving electrolyte matrix cracking resistance have been proposed. A number of these approaches are based on incorporating secondary reinforcing phases into the matrix. These secondary reinforcements have the form of large particles (see U.S. Pat. No. 4,322,482 and U.S. Pat. No. 4,538,348), fibers (see U.S. Pat. No. 5,316,555), metal wire mesh screens (see U.S. Pat. No. 3,432,363) and metal powders (Al, Zn) (see U.S. Pat. No. 3,466,197). Moreover, they act as crack attenuators and deflectors to slow crack propagation or to enhance the bonding between the ceramic support materials, resulting in improved matrix strength and toughness.
Where the reinforcements act as crack deflectors, (i.e., fibers, screens, particles), unsintered ceramic particles (including the secondary reinforcements) in the matrices are merely bonded with the impregnated carbonate of low strength. As a result, thermal stress alone can still generate microcracks in the matrix. Microcracks can also be generated by mechanical stress, particularly during the start-up after binder burnout. Although these microcracks are not of a size which allows fuel and oxidant gas cross leakage during initial fuel cell operation, they can eventually propagate to larger sizes, causing increased cross leakage after several thermal cycles.
The aforesaid microcracking can be reduced by the use of reinforcements which enhance bonding. Thus, dispersed metal powder phase (Al, Zn) reinforcements can sinter to provide higher strength during both start-up and operation. A higher strength is essential for reducing microcracking. The low melting points of the metal phases, .about.660.degree. C. for Al and .about.419.degree. C. for Zn are required for producing the sintering.
While the use of metal bonding reinforcements thus increases matrix strength, it would be advantageous to provide further procedures and matrix configurations which result in added strength for the matrices.
It has also been known to incorporate into the electrolyte of carbonate fuel cells alkaline earth additives (Mg.sup.++, Ca.sup.++, Sr.sup.++, Ba.sup.++). These additives are homogeneously mixed with the electrolyte and flow into the matrix with the electrolyte during start-up of the cell.
The presence of the additives suppresses dissolution of NiO in the cell cathode electrodes. If such dissolution is allowed to occur it can cause metallic deposition in the matrix which can result in cell shorting.
While incorporation of such alkaline earth additives in the electrolyte is thus desirable, the quantity of additive has been limited. Additionally, the uniformity of the additive when resident in the matrix has also been limited.
It is an object of the present invention to provide an electrolyte matrix of increased strength.
It is a further object of the present invention to provide a method for fabricating such increased strength electrolyte matrix.
It is an additional object of the invention to provide an electrolyte matrix having increased amounts and improved uniformity of alkaline earth additives.